Acorn System 4 Capacitor Replacement Guide

This guide covers recapping the Acorn System 4 (1980). As a Eurocard rack it has per-card capacitors (a regulator electrolytic plus ceramic decouplers on each card), and as a cased machine it also has bulk smoothing electrolytics in its integral mains power supply.

Per-card capacitors

On each Eurocard the recap target is the +5 V regulator electrolytic; the ceramic decouplers are stable and left in place. Read each electrolytic's value from the card and its circuit diagram and fit a 105 °C low-ESR part of equal capacitance and equal-or-higher voltage, observing polarity.

• CPU card — the Acorn System 1 card: a 15 µF/16 V electrolytic (see Acorn System 1 Capacitor Replacement Guide).
• VDU, RAM, floppy-controller, Econet and interface cards — each has its own regulator electrolytic plus decouplers.

⚠️ Mains power supply

The System 4's integral PSU is mains-powered and contains the bulk smoothing electrolytics (and mains-side suppression parts). Disconnect the mains lead and discharge the bulk capacitor before any PSU work. Recap the PSU only if competent with mains supplies; otherwise replace the unit. Read each value from the fitted part.

Procedure (per card)

1. Power off; remove the card from the backplane.
2. Record each electrolytic's value, voltage and polarity.
3. Desolder, clean the holes, fit the new part with correct polarity, solder and trim.
4. Verify +5 V on the card and refit.

Related pages

• Acorn System 4
• Acorn System 4 Maintenance Guide
• Acorn System 4 Troubleshooting Guide
• Acorn System 1 Capacitor Replacement Guide · Capacitor Failure Symptoms